1. Field of the Invention
The present invention relates to an analog to digital converter, and more particularly to an analog to digital converter having high linearity.
2. Description of the Related Art
Consumers increasingly rely on digital resources provided by electronic devices such as cellular telephones, digital cameras, or portable and handheld digital electronic devices. The electronic devices process and/or produce both digital and analog signals. Meanwhile, the demand for faster transmission of digital data is increasing, along with increasing demand for applications such as wireless networks, downloadable digital music devices, digital movie devices, and others.
Electronic devices require the receipt of analog signals, which are then converted to digital signals, referred to as analog to digital (A/D) conversion. The electronic devices include appropriate circuitry to perform the A/D conversion to perform digital signal processing.
The pipelined architecture for Analog-to-Digital Conversion (ADC) rely on the concept of simultaneous data sub-conversion in multiple stages in order to progressively refine the digital representation of an analog signal. There are two fundamental approaches to pipelined ADC: a switched-capacitor, and a switched-current approach. In both of the approaches, the ADC apparatus is negatively influenced when components employed during stages of the pipelined ADC are mismatched.
FIG. 3 is a schematic diagram showing an analog-to-digital conversion error in a conventional analog to digital converter. The vertical axis is the DNL (differential nonlinearity) value which is the difference between an actual output digital value and an ideal output digital value. The horizontal axis is the ideal output digital value. In FIG. 3, two peaks of the DNL value occur at the ideal output digital values 96 and 160. Thus, the output digital value is 97 and 161, not 96 and 160. The DNL peaks are generated due to capacitor mismatch, operation amplifier error or settling error in the analog to digital converter. Thus, performance and linearity of the analog to digital converter is decreased.